Influenza vaccines, commonly known as flu shots, are updated twice a year because the virus mutates quickly, and they offer modest to high but variable protection annually. The WHO, CDC, and ECDC recommend yearly vaccination for nearly everyone over six months, with special focus on pregnant women, the elderly, children aged 6 months to 5 years, and those with underlying health issues. While generally safe—with mild effects like injection-site soreness or fever in 5-10% of children—the vaccine carries a rare link to Guillain-Barré syndrome, occurring in about one per million doses in older adults. Vaccination began experimentally in the 1930s, with the first approved inactivated vaccines used by the US military in 1945 during World War II. Modern versions come in inactive, weakened, or recombinant forms given via injection, nasal spray, or intradermal routes, with FDA-approved cell-based and recombinant vaccines arriving in 2012 and 2013 respectively. A notable historical moment was the 1976 swine flu scare, when President Gerald Ford's rushed vaccination program faced delays and public trust issues, though today's methods have evolved far beyond the original egg-based technology.